1. Field of the Invention
The present invention relates to an X-ray projection exposure apparatus which is suitable for use in transferring circuit patterns on a mask (also called "reticle") onto a substrate, such as a wafer, etc., and more particularly, to an X-ray projection exposure apparatus having a reflective type focusing optical system of a mirror projection system, such as an X-ray optical system, etc.
2. Discussion of the Related Art
Conventionally, in a exposure apparatus used for semiconductor manufacture, circuit patterns formed on a mask (photo-mask) are projected and transferred onto a photosensitive substrate, such as a wafer, etc., via a focusing optical system. The photosensitive substrate is coated with a resist, and the resist is exposed to form a resist pattern.
The resolving power W of the exposure apparatus is determined mainly by the wavelength .lambda. of the exposing light and the numerical aperture NA of the focusing optical system, and is expressed by the following equation: EQU W=k.lambda./NA (k: constant) (1)
Accordingly, in order to improve the resolving power, it is necessary to either shorten the wavelength or increase the numerical aperture. Currently, exposure apparatus used in the manufacture of semiconductor devices normally employs the i-line having a wavelength of 365 nm, so that a resolving power of 0.5 .mu.m is obtained at a numerical aperture of approximately 0.5. Since increasing of the numerical aperture is difficult from the standpoint of optical design, it will be necessary in the future to shorten the wavelength of the exposing light. An example of exposing light having a wavelength shorter than the i-line is excimer laser light. The wavelengths are 248 nm for KrF and 193 nm for ArF, respectively. A resolving power of 0.25 .mu.m can be achieved in the case of KrF, and a resolving power of 0.18 .mu.m can be achieved in the case of ArF. Furthermore, if X-rays having an even shorter wavelength are available as exposing light, a resolving power of 0.1 .mu.m or less should be achieved at a wavelength of 13 nm, for example.
A conventional exposure apparatus is constructed mainly of a light source, an illumination optical system, and a projection and focusing optical system. The projection and focusing optical system is constructed from a plurality of lenses or reflective mirrors, etc., and is arranged so that the pattern on the mask if focused on the wafer.
To obtain the desired resolving power, it is necessary that at least the focusing optical system be essentially free from any aberrations. If any aberrations are present in the focusing optical system, the desired cross-sectional shape in the resist pattern cannot be obtained, and accordingly, adverse effects, such as image distortion problem, may arise in the processes due to such inadequate exposure.
Furthermore, in the conventional semiconductor exposure apparatus, a position detection device (also referred to as an "alignment device") is provided so that a resist pattern can be formed in a pre-determined position on the wafer with respect to existing circuit patterns on the wafer. The positions of the mask and the wafer are detected by the alignment device, and the respective positions of the wafer and the mask are adjusted by a wafer stage and a mask stage so that a reduced image of the mask pattern is focused at a prescribed position on the surface of the wafer.
The alignment device may be an optical detection device. The optical detection device illuminates alignment marks on the wafer with illuminating light and detects the reflected light or the like by a photo-detector, for example. When the wafer position changes, the signal output from the detector also changes, so that the wafer position can be measured. Similarly, in the case of the mask, the position of the mask can be detected by illuminating alignment marks on the mask with illuminating light, and then detecting the reflected light or the like by a photo-detector.
Such an alignment device can detect the positions of the alignment marks on the wafer and the mask with a high degree of precision. Accordingly, alignment of the mask and wafer can accurately be performed. In the conventional exposure apparatus, the alignment device is disposed between the focusing optical system and the wafer and/or between the focusing optical system and the mask.
FIG. 4 schematically shows main elements of a conventional exposure apparatus using the i-line. The conventional exposure apparatus includes a light source, an illumination optical system (not shown in the figures), a stage 15 for holding mask 14, a projection and focusing optical system 13, a stage 17 for holding wafer 16, and alignment devices 18. A mask pattern, which is equal in size to the pattern that is to be drawn on the wafer, or which is to be reduced upon exposure, is formed on the mask 14. The projection and focusing optical system 13 is constructed of a plurality of lenses, etc., and is arranged to focus the image of the pattern on the mask 14 onto the wafer 16. The focusing optical system has a field of view of about 20 mm in diameter so that the entire mask pattern can be transferred to the surface of the wafer 16 in a single transfer. The positions of alignment marks on the mask and the wafer are detected by the alignment devices 18 (position detection device).
As described above, in the conventional exposure apparatus using the i-line or the like, the projection and focusing optical system can be constructed of lenses. Accordingly, an optical system having a field of view of 20 mm square or even greater can be designed. Thus, a desired region (e.g., an area corresponding to two (2) semiconductor chips) can be exposed at one time.
On the other hand, in the design of a focusing optical system for X-rays, the field of view needs to be reduced, and therefore, the desired large region cannot be exposed at one time. To cope with this problem, a proposed design of the X-ray exposure apparatus utilizes a scanning scheme in which a semiconductor chip with an area of 20 mm square or greater is exposed using a focusing optical system with a small field of view by synchronously scanning the mask and the wafer during exposure. By using such a method, it is possible to expose the desired large exposure region by an X-ray projection exposure apparatus.
For example, in the case of exposure by X-rays with a wavelength of 13 nm, it is possible to form the exposure field of view of the projection and focusing optical system as an annular band like shape, so that a high resolving power can be obtained.
FIG. 5 schematically shows a proposed design of an X-ray projection exposure apparatus. The apparatus includes an X-ray source 1, an X-ray illumination optical system 2, a stage 5 for holding mask 4, an X-ray projection and focusing optical system 3, and a stage 7 for holding wafer 6. A pattern which is equal in size to the pattern that is to be drawn on the wafer, or which is to be reduced upon exposure, is formed on the mask 4. The projection and focusing optical system 3 includes a plurality of reflective mirrors, etc., and is designed to focus the image of the pattern on the mask 4 onto the wafer 6. The focusing optical system 3 has an annular band like field of view, and a portion of the mask pattern in an annular band like region is transferred onto the wafer 6. During exposure, the mask 4 is illuminated with X-rays 11, and the reflected X-rays 12 are projected towards the wafer 6 through the X-ray projection and focusing optical system 3. Mask 4 and wafer 6 are synchronously scanned at respective constant speeds to expose a desired region (e.g., a region corresponding to one semiconductor chip).
In general, when an alignment device is installed in an X-ray projection exposure apparatus, a light source and an illumination system, which illuminate alignment marks on the mask and/or the wafer are required. The installation of such a light source and illumination system in the exposure apparatus involves difficulties in terms of layout, and also increases costs. This is particularly a problem in the design of an X-ray projection exposure apparatus. In an X-ray projection exposure apparatus, some of the reflective mirrors constituting the focusing optical system are disposed in close proximity to the wafer due to various optical design constraints. Accordingly, it is difficult to install the optical system of the alignment device between the focusing optical system and the wafer.
In this connection, the following two points are noteworthy: (1) If the position of the reflective mirror closest to the wafer among the reflective mirrors constituting the focusing optical system is removed from the wafer in order to create a sufficient gap between the wafer and the reflective mirrors to install the optical system of the alignment device, the focusing performance of the focusing optical system suffers, and the desired fine pattern cannot be projected; and (2) If the reflective mirror closest to the wafer among the reflective mirrors constituting the focusing optical system is made thinner in order to increase the above-mentioned gap to install the optical system of the alignment device, the rigidity of the mirror drops, and it becomes difficult to manufacture such a mirror with high accuracy. In other words, these simple methods of increasing the gap between the wafer and the reflective mirrors to install the optical system of the alignment device would sacrifice the optical performance of the focusing optical system.